User input passed to printf pattern should be sanitized

[fstark]

Description:

Pattern specifications from the command line are passed to printf without any sanitization, leading to crashes.

How to reproduce:

./flimmaker '%s.pgm'

Result:

Segmentation fault (core dumped)

Expected Result:

Empty movie or user-friendly error message.

[fstark]

Fixing this would either be real difficult, or make the system less powerful. Wontfix.